Inorg. Chem. 2006, 45, 6592−6594
Trinuclear Gold(I) Triazolates: A New Class of Wide-Band Phosphors
and Sensors
Chi Yang,† Marc Messerschmidt,‡ Philip Coppens,*,‡ and Mohammad A. Omary*,†
Department of Chemistry, UniVersity of North Texas, Denton, Texas 76203, and Department of
Chemistry, UniVersity at Buffalo, State UniVersity of New York, Buffalo, New York 14260-3000
Received May 30, 2006
A new cyclic gold(I) triazolate trimer, [Au(3,5-i-Pr2Tz)]3 (1), exhibits
fully overlapping aurophilically bonded dimer-of-trimer units that
lead to multiple phosphorescence bands in both the solid state
and solution. The conformation of the hexanuclear unit exhibits
reversible interconversion between C2 and D3 effective symmetries,
depending on the crystal temperature or solution concentration,
the variation of which leads to isoemissive and isosbestic points.
Solutions of 1 exhibit remarkable quenching properties that
demonstrate molecular recognition with high selectivity and
hypersensitivity for some reagents, as influenced by protonation
via Brønsted acids, π intercalation, and/or energy transfer. The
quenched phosphorescence of 1 by acetic acids can be regenerated by NEt3.
Recent increasing interest in phosphorescent metal complexes has been driven by their promising photonic
applications.1-4 Particularly attractive are cyclic trinuclear
AuI complexes because they exhibit bright phosphorescence
with a short lifetime in the solid state at room temperature,
tunable emission colors across the visible region, reasonable
charge-transport properties, and sensitivity to multiple
* To whom correspondence should be addressed. E-mail: [email protected]
(M.A.O.), [email protected] (P.C.).
† University of North Texas.
‡ University at Buffalo, State University of New York.
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6592 Inorganic Chemistry, Vol. 45, No. 17, 2006
stimuli.4-6 The significance of cyclic trinuclear d10 complexes
also spans multiple fundamental areas, including groundstate metallophilic bonding, M-M excimeric bonding, supramolecular assemblies, and metalloaromaticity.4-7
Structural and photophysical studies for cyclic trinuclear
AuI complexes have thus far involved pyridiniate, imidazolate, pyrazolate, and carbeniate4-6 but not triazolate
bridging ligands.8a Compared to imidazole and pyrazole,
1,2,4-triazole is expected to possess a similar coordination
affinity to soft metal ions (e.g., AuI) but with a smaller
extent of electron donation because of the inductive effect
of the additional nonbridging N atom. Hence, we decided
to explore this new class of cyclic trinuclear metallomacrocycles of triazole with d10 metal ions, [MTz]3. Derivatives
of 1,2,4-triazole show diverse coordination modes and
form coordination networks with metal ions.8 The exo N
atoms in [AuTz]3 can serve as sites for H bonding or
coordination to metal ions. Thus, [AuTz]3 can be potentially
used as building blocks for supramolecular chemistry as
well as potential sensors for metal coordination, H bonding, and/or pH detection. Herein, we report the synthesis, structure, and unusual luminescence properties of
cyclo-tris[µ N,N -3,5-diisopropyl-1,2,4-triazolategold(I)],
[Au(3,5-i-Pr2Tz)]3 (1).
The synthesis of 1 involves the addition of an excess of
freshly distilled anhydrous NEt3 to a methanol solution of
gold(tetrahydrothiophene)chloride and 3,5-diisopropyl-4H1,2,4-triazole. After stirring for 30 min, the product was
precipitated as a white solid in ∼80% yield.
(5) (a) White-Morris, R. L.; Olmstead, M. M.; Attar, S.; Balch, A. L.
Inorg. Chem. 2005, 44, 5021. (b) Kishimura, A.; Yamashita, T.; Aida,
T. J. Am. Chem. Soc. 2005, 127, 179. (c) Olmstead, M. M.; Attar, S.;
Balch, A. L. Inorg. Chem. 2005, 44, 5021.
(6) (a) Omary, M. A.; Rawashdeh-Omary, M. A.; Gonser, M. W. A.;
Elbjeirami, O.; Grimes, T.; Cundari, T. R.; Diyabalanage, H. V. K.;
Gamage, C. S. P.; Dias, H. V. R. Inorg. Chem. 2005, 44, 8200. (b)
Yang, G.; Raptis, R. G. Inorg. Chem. 2003, 42, 261.
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2005, 249, 2740.
(8) (a) Minghetti, G.; Banditelli, G.; Bonati, F. Inorg. Chem. 1979, 18,
658. (b) Nomiya, K.; Noguchi, R.; Ohsawa, K.; Tsuda, K. J. Chem.
Soc., Dalton Trans. 1998, 24, 4101. (c) Haasnoot, J. G. Coord. Chem.
ReV. 2000, 200-202, 131 and references cited therein.
10.1021/ic060943i CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/26/2006
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cm-1
Figure 1. ORTEP plot for the dimer-of-trimer structure of 1 at 95 K (left)
and RT (right). For clarity purposes, the i-Pr groups are omitted and the
thermal ellipsoids are shown at 50% (95 K) and 33% (RT) probability.
Figure 2. Photoluminescence spectra for crystals of 1 (λex ) 280 nm) vs
temperature. The inset shows excitation spectra monitoring at λem ) 650
nm. Sky-blue traces in the emission and excitation illustrate isoemissive
and isosbestic points, respectively, as indicated by the arrows.
The structure of 1 has been determined at 95 K and room
temperature (RT) (Figure 1). The complex crystallizes in the
C2/c space group with the asymmetric unit containing one
independent [Au(3,5-i-Pr2Tz)]3 trimer molecule, which dimerizes into a fully overlapping dimer-of-trimer prismatic unit
via three AuI‚‚‚AuI aurophilic bonds. The marked thermal
contraction of the structure upon cooling of the crystal is
evidenced by changes at RT vs 95 K in the crystal density
(2.209 vs 2.342 g‚cm-3) and the cell volume (6300.15 vs
5943.22 Å3). Although the crystal packing mode is similar
at both RT and 95 K, the conformation of the dimer-of-trimer
unit is remarkably different, in which the Au‚‚‚Au intertrimer
distances of 3.19, 3.55, and 3.55 Å at 95 K and 3.46, 3.42,
and 3.42 Å at RT suggest C2 and D3 effective symmetries,
respectively.
Figure 2 shows the photoluminescence spectra for crystals
of 1 vs temperature. The emission consists of a higher-energy
(HE) band in the UV region and two lower-energy emissions
with maxima near 650 nm (LE1) and 750 nm (LE2) at T <
180 K, whereas at T g 180 K, only the LE2 band dominates.
Upon heating, the HE band maintains its structured profile
but decreases in intensity and ultimately disappears around
160 K, whereas the intensity of the LE1 band decreases
concomitantly with an increase in the intensity of the LE2
band.
The HE band for crystals of 1 exhibits a well-resolved
vibronic structure with an average spacing of 1350 ( 40
, which clearly correlates with IR frequencies of 1 and
the free ligand (see the Supporting Information). The decay
times for this band are 68.5 ( 0.6 µs at 4 K and 28.6 ( 0.1
µs at 77 K. These observations suggest that the HE band is
due to a triplet state with a strong ligand character in the
lower singly occupied molecular orbital.
The LE1 and LE2 bands exhibit a broad unstructured
profile suggestive of excimeric emissions from a dimer-oftrimer unit, in analogy to similar bands exhibited by other
coinage-metal cyclic trimers;6 we have recently verified such
an assignment for crystals of [Cu(3,5-(CF3)2Pz)]3 by timeresolved X-ray diffraction.9 Each of the LE1 and LE2 bands
in crystalline 1 decays with an identical single-exponential
time constant at the same temperature. Whether λem is
monitored at 650 or 750 nm, the decay times are 62.3 (
0.5, 27.2 ( 0.3, and 19.1 ( 0.1 µs at 4, 77, and 300 K,
respectively. The excitation profiles are very similar for the
LE1 and LE2 bands and are different from that for the HE
band. These data suggest an assignment to two phosphorescent excimeric states for the LE1 and LE2 bands, which are
coupled to one another but are uncoupled from the HE state.
The existence of multiple unstructured LE emissions at 95
K or lower temperatures but only one such a band at RT is
in agreement with the aforementioned crystal structures
showing C2 and D3 dimer-of-trimer effective symmetries at
95 K and RT, respectively, because more bands are expected
at a lower symmetry. Figure 2 illustrates an isoemissive
point10 at 684 nm (emission spectra) and an isosbestic point
at 293 nm (excitation spectra) along with the progressive
increase in the LE2/LE1 intensity ratio upon heating of the
crystal between 40 and 100 K. This observation, together
with the significant change in the luminescence excitation
profile (Figure 2, inset), suggests a structural change of the
effective chromophore that we interpret as interconversion
between C2 and D3 effective symmetries for the dimer-oftrimer unit by integrating these spectral observations with
the temperature-dependent structural findings above. The
gradual red shift in the LE2 emission peak upon cooling (λmax
shift ∼ 725 f 755 nm) is also consistent with the shorter
intertrimer Au‚‚‚Au distance at 95 K than at RT. We are
aware of only one similar precedent, reported recently by
the Balch group for [µ3-S(AuCNC7H13)3](SbF6),11 in which
a reversible temperature-induced structural change resulted
in alterations of both aurophilicity and luminescence.
Although luminescence from solids or rigid glasses of twocoordinate AuI complexes is well-known, it is quite uncommon that fluid solutions of such complexes exhibit Aucentered emissions at RT. This is particularly unusual for
emissions arising from ligand-unassisted AuI‚‚‚AuI interactions.12 Surprisingly, the unstructured LE1 and LE2 bands
of 1 are also observed in CH2Cl2 solutions at RT, even at
(9) Vorontsov, I. I.; Kovalevsky, A. Yu.; Chen, Y.-S.; Graber, T.;
Novozhilova, I. V.; Omary, M. A.; Coppens, P. Phys. ReV. Lett. 2005,
94, 193003.
(10) For an example illustrating the significance of isoemissive points in
photophysics, see: Kowalczyk, A.; Boens, N.; Bergh, V. V. D.;
Schryver, C. D. J. Phys. Chem. 1994, 98, 8585.
(11) Gussenhoven, E. M.; Fettinger, J. C.; Pham, D. M.; Malwitz, M. M.;
Balch, A. L. J. Am. Chem. Soc. 2005, 127, 10838.
Inorganic Chemistry, Vol. 45, No. 17, 2006
6593
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Figure 3. Top: Area-normalized emission spectra of 1 in CH2Cl2 at RT.
Concentrations (×10-4 M) are 88, 44, 11, 2.8, 0.68, and 0.16 for traces
1-6, respectively (λex ) 270 nm). Bottom: Change in the emission
spectrum of a 3.0-mL 5.0 × 10-3 M CH2Cl2 solution of 1 (a) upon the
addition of 0.20 mL of CH3COOH (b), 0.23 mL of benzene (c), 0.3 mg of
resorcinol (d), or 7.0 µL of CF3COOH (e). Gray traces show the recovery
of the quenched emission in trace b by successive additions of aliquots of
NEt3 to a total of 0.21 mL (f). The added reagents represent 3.5, 2.6, 0.0027,
0.094, and 1.49 mmol for traces b-f, respectively.
Scheme 1
low concentrations (Figure 3). The lifetime for a 1.0 × 10-3
M solution is 2.06 ( 0.02 µs. These spectral observations
suggest that the aurophilically bonded dimer-of-trimer unit
is preserved in a CH2Cl2 solution. Though persistence of
unassisted d10‚‚‚d10 interactions in solution has been encountered earlier,6,12 the unusual behavior herein is the obserVation
of AuI-AuI bonded excimer phosphorescence in dilute
solutions at RT for 1 in CH2Cl2. The LE1/LE2 intensity ratio
is concentration-dependent. As shown in Figure 3 (top),
progressive dilution of a 8.8 × 10-3 M solution results in a
gradual decrease of the LE2 band intensity accompanying
the increasing dominance of the LE1 band. More importantly,
the area-normalized spectra shown in Figure 3 (top) display
an apparent isoemissive point at 686 nm, reminiscent of the
crystal emission behavior upon cooling. Both situations are,
therefore, attributed to a conformation change between C2
and D3 effective symmetries in the Au6 unit.
The concentration-dependent solution emission of 1 encouraged us to explore its potential sensor action. As demonstrated in Figure 3 (bottom), the emission of 1 in CH2Cl2
can be quenched by the addition of CH3COOH or benzene
and, more efficiently, by CF3COOH or resorcinol. No
quenching is observed upon the addition of CHCl3, methanol,
diethyl ether, or n-hexane. Most importantly, the quenched
emission of 1 by CH3COOH or CF3COOH can be recovered
by the addition of NEt3, whereas the quenching by benzene
and resorcinol cannot be reversed using NEt3. Because the
solution emission of 1 is due to intertrimer interactions in
dimer-of-trimer units, the quenching will be caused either
by loss of the excitation energy or by dissociation to two
monomer-of-trimer units in solution. For the latter case,
dissociation can be induced by guest intercalation between
the two planar trimers in the dimer-of-trimer unit or by
protonation or H bonding of the exo N atoms of the triazole
ring. The quenching of the emission of 1 by CH3COOH or
CF3COOH and its reversible restoration by NEt3 suggest that
protonation likely induces repulsion between the two bound
trimers, leading to dissociation to two separate monomerof-trimer units.13 In the case of benzene, π intercalation is
possible between the two planar trimers in the dimer-oftrimer unit. However, the hypersensitive quenching of the
emission of 1 by resorcinol (a weak Brønsted acid) suggests
that resorcinol also serves as an energy trap, which is facilitated by H bonding and/or π interactions in the cooperative
quenching mechanism. Further work to quantify the quenching processes and delineate their multiple mechanisms will
follow.
In conclusion, 1 represents a new class of phosphorescent
complexes with a remarkable photophysical behavior, several
aspects of which are summarized in Scheme 1. Work is
underway to further characterize 1 and other coinage-metal
cyclic trinuclear triazolates and assess their potential in
photonic and sensor applications.
Acknowledgment. This work is supported by the National Science Foundation (CAREER Award, Grant CHE0349313, to M.A.O.), the Robert A. Welch Foundation
(Grant B-1542 to M.A.O.), and the U.S. Department of
Energy (Grant DE-FG02-02ER15372 to P.C.).
Supporting Information Available: Crystallographic data
(CIF) and further experimental details (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
IC060943I
(12) (a) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler,
J. P., Jr. J. Am. Chem. Soc. 2001, 123, 11237. (b) Tang, S. S.; Chang,
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6594 Inorganic Chemistry, Vol. 45, No. 17, 2006
(13) Note that emission quenching of 1 in CH2Cl2 by CF3COOH also
changes the spectral profile: trace e in the bottom of Figure 3 is similar
to the emission profile of 1 in its C2 conformation, which exists in
dilute CH2Cl2 solutions (see Figure 3, top). The detailed mechanism
will be addressed in the full paper.